Relationships do act as triggers for growth. However triggers come from all sorts of situations..not just with friendships or relationships.
There are also many people who cannot be by themselves - whether alone, or in silence. Mind you until we learn to quieten our mind, there might be several things going around and around in the brain..We often manufacture thoughts through worry, What ifs? Should I? etc. We 'create' these - so why not solutions too? That inner voice, reveals a lot of what we need to do to 'know ourself'.
How many people actually take note of the positive inner voice? I beleive that negative critical voice starts from an external source..and those who struggle with this, is because they mostly play it over and over...Again, this is through experiencing internal contradictions and working on myself to bring about change...also helping a great many others feed the positive!

I would also love to have grown as a person in a close loving intimate relationship...obviously was not meant to happen that way lol.
Now I have a greater understanding of why my life has been as it has...and it has been with continual learning. Had I settled down, had children, domestic chaos/bliss etc, ..different experiences and lessons will have come. Lots of heartache here...though it forced me to study, to try understand more, and to search within.

Mindfulness, as in being in the present is not always fully applied because when we have chattering going on in our head and our own 'to do list' going on. How can we possibly be fully engaging? Psychotherapists/Counsellors might apply this to some degree. Some actually are not fully present and have their own stuff going on in their head. Sad but true. Though myself personally, I work with the whole person, not just the issues presented. This does not always mean sticking with the present moment - though past, internal process, external factors are usually discussed from the present tense. We all work differently.

Interesting point. I am talking from a personal philosophical standpoint now...
I think we encounter mirrors in many people, not just partners and each person teaches us something new about ourselves. In my case I was never good at expressing my personal feelings 'of love' and was being true to myself in the past because the love I have felt has not been deep enough to make a committment.
The 'monsters' you mention are mostly insecurity based or triggered from the past...This is what I understand. Deep soul searching when we react in a certain way will reveal the root...Asking ourselves why we are feeling/thinking this way, can be revealing. We cannot be held responsible for other peoples reactions..however we can be responsible for our own and our behaviour - all complex stuff, I know.
Again from my personal standpoint and having done a lot of work on myself (with much more to do as we are constantly growing and evolving) Whether you are living a life of singledom, solitude, in a loving partnership or a volatile relationship....all are situations that CAN enable our journey of self-discovery. However, if you are keen to be more authentic and your partner has say superficial priorities - It could be other issues apply, conflict will emerge/reactions etc. When you deny your own feelings, self-expression and needs being met that is when our growth and personal development is hindered. On the other hand you could be in a very destructive situation and grow tremendously from the experience. There are no cut and dry rules - we all have different lessons we will learn during our lifetime...We are all on that journey of self-discovery...However there are layers of awareness that also apply. Different people have different priorities in life. It is good to see you explore other peoples experiences and ideas...though YOUR personal experience is your most valuable life philosophy. Know Thyself is the most powerful key to truth and wholeness. That is all that is required!

When everything falls apart around you, this is an ideal time to re-create your life how you want it to be. Start with writing a list...and focus on you, what would you like to personally do in life?. It might be that you relied on someone else and they let you down in some way....As we grow we realise we need to rely on ourselves. Of course most of us want love, cuddles and affection - but not to be dependant on that. The reason I say this is we could find ourselve alone..and what then, our foundation is shaken up again.
Take one day at at time - solid foundations do not happen overnight. Enjoy your journey of self-discovery.